Reinforcement from most drugs, including nicotine, depends largely on their interoceptive stimulus effects. Although traditional subjective measures may help, clearer understanding of these effects, and factors that influence them, requires use of behavioral drug discrimination procedures. Knowledge of the discriminative stimulus effects of nicotine may be critical in understanding its dependence potential and in evaluating new treatments for nicotine dependence. In the first 2.5 years of this project, we have established a procedure for assessing nicotine discrimination I humans using a measured-dose nasal spray delivery system and have begun to examine factors influencing this discrimination, such as between-subjects differences (e.g.. Sex, smoking status) and within-subjects factors (e.g. training conditions). The research proposed in the present application directly extends this line of investigation to key questions concerning the relationship between nicotine discrimination and reinforcement, mechanisms underlying nicotine discrimination, and common environmental factors which may, acutely influence nicotine discrimination. More specifically, we plan to: 1) identify the lowest reliably discriminable nicotine doses (discrimination threshold) in smokers and nonsmokers; 2) compare effects of central plus peripheral nicotine blockage (via mecamylamine) with effects of peripheral blockade only (via trimethaphan) to verify that nicotine discrimination is dependent on its central effects; 3) systematically evaluate alterations in the discriminative stimulus effects of nicotine due to situational conditions that commonly accompany nicotine intake via smoking in the natural environment, including concurrent drug consumption (alcohol, caffeine) and concurrent engagement in light physical activity; and 4) determine the relationship between nicotine choice (a measure of reinforcement) and nicotine discrimination following each of these manipulations. Findings from the first study may assist efforts to identify thresholds for nicotine reinforcement and development of dependence, an objective with important implications for prevention and treatment of nicotine dependence. Results of this program may also ultimately assist in the development of new treatments for smoking cessation by: providing a means for testing new medications to alter nicotine's reinforcing effects; confirming that these effects are centrally-mediated; and by determining the influence of situational factors of these effects, thus providing a better understanding of how these factors may impact on nicotine reinforcement, maintenance of smoking, and smoking relapse.